Conventionally, hard plaster is produced from raw plaster (dihydrates). Various methods are known for this purpose. A first method operates with aqueous plaster suspensions which are treated in an autoclave at temperatures of 105° C. to 165° C. and with an overpressure of easily 1 to 6 bar. There is also a pressureless method in which a plaster suspension in sulfuric acid is converted by means of other acids at temperatures of below 105° C. Moreover, there are also what are known as quasi-dry methods (DE-A-3819652, DE-A-3819653, EP-A-0572781), in which dehydrated plaster having a relatively low residual moisture of less than 20% is converted in autoclaves at temperatures between 105° C. and 165° C. and with an overpressure of 1 to 6 bar. What all these methods have in common is that the raw material (dihydrate) is converted directly into hard plaster (alpha-hemihydrate), wherein aggregates may be required where appropriate, such as, for example, a 5% fraction of beta-hemihydrate, also being required, where appropriate. What the methods also have in common is that they are complicated and entail high costs both in terms of energy and in terms of the materials used (acids).
Conventionally, hard plaster is produced from raw plaster (dihydrates). Various methods are known for this purpose. A first method operates with aqueous plaster suspensions which are treated in an autoclave at temperatures of 105° C. to 165° C. and with an overpressure of easily 1 to 6 bar. There is also a pressureless method in which a plaster suspension in sulfuric acid is converted by means of other acids at temperatures of below 105° C. Moreover, there are also what are known as quasi-dry methods (DE-A-3819652, DE-A-3819653, EP-A0572781), in which dehydrated plaster having a relatively low residual moisture of less than 20% is converted in autoclaves at temperatures between 105° C. and 165° C. and with an overpressure of 1 to 6 bar. What all these methods have in common is that the raw material (dihydrate) is converted directly into hard plaster (alpha-hemihydrate), aggregates, such as, for example, a 5% fraction of beta-hemihydrate, also being required, where appropriate. What the methods also have in common is that they are complicated and entail high costs both in terms of energy and in terms of the materials used (acids).
Further, a method is known in which beta-plaster is subjected to artificial aging within the framework of posttreatment, in order to increase mechanical pressure resistance (DE-B1771625). The method provides first for calcining into conventional stucco plaster (beta-plaster), alpha-hemihydrate being partially generated from the beta-hemihydrate in the subsequent process. In order to achieve this, water is added during posttreatment, specifically in a sufficient amount to lower the temperature of the suspension in a range below the calcining temperature. Rehydration of the beta-hemihydrate into dihydrate is thereby set in motion. Subsequently, the cooled plaster is heated again to a temperature above the calcining temperature, so that, from the dihydrate rehydrated by cooling, hemihydrate is formed once again, specifically in the alpha modification. One disadvantage of this method is that that fraction of the alpha modification which is achieved is not very high, and therefore it has only low efficiency, and, moreover, the product still has relatively high BET (“Brunauer-Emmett-Teller”) values which are considerably above those of conventional hard plaster.
For the selective production of beta or alpha plaster, a method is known (DE-A-3738301) in which the dihydrate is acted upon in a calciner with hot fluidizing gas. Whether beta plaster or alpha plaster is formed is controlled by setting the temperature and pressure. For alpha plaster, temperatures of above 250° C. are required in the calciner, with a pressure of 2 to 6 bar. The method consequently resembles the known use of autoclaves and shares their disadvantages. Also, purity in terms of the fraction of alpha modification is only unsatisfactory.
A multistage plant for calcining is known from US 2008152559. A plurality of the reactors are connected in series in order to burn the plaster material in batches to form anhydrite. Hot steam can be supplied for heating purposes. The reactors can be provided with integrated grinding plants in order to grind the anhydrite into as uniform a small particle size as possible. The plant is designed for temperatures of above 550° C. and pressures of 70 bar. The anhydritic (“deadburnt”) alpha plaster thus generated does not have the advantageous properties of alpha-hemihydrate plaster.
Finally, a method for the production of alpha plaster is known, in which beta plaster is converted directly into alpha plaster (DE-A-2049028). For this purpose, briquets composed of beta plaster mixed with water are converted in an autoclave into alpha plaster over several hours, the addition of aggregates being provided.